On the use of 17 O NMR for understanding molecular and silica-grafted tungsten oxo siloxide complexes.

17 O-labelled tungsten siloxide complexes [WOCl 2 (OSi t Bu 3 ) 2 ] (1-Cl) and [WOMe 2 (OSi t Bu 3 ) 2 ] (1-Me) were prepared and characterized by 17 O MAS NMR, with input from theoretical calculations of NMR parameters. Guidelines linking 17 O NMR parameters and the coordination sphere of molecular and silica-grafted tungsten oxo species are proposed. The grafting of 1-Me on SiO 2-700 afforded material 2, with surface species [(SiO)WOMe 2 (OSi t Bu 3 )] as shown by elemental analysis, IR and 1 H and 13 C MAS NMR. The DFT calculations of the grafting mechanism are in line with the observed reactivity. They indicate the occurrence of several isomeric species of close energy for the grafted W centers, precluding efficient 17 O MAS NMR studies. The lack of catalytic activity in olefin metathesis and ring-opening olefin metathesis polymerization indicates that initiation by α-H elimination is not operative in 2, contrary to related tungsten surface species, which illustrates the crucial influence of the nature of the metal coordination sphere.